Thanks for your question. The first thing to say is that you’re not alone in your struggle with lust. This is an area that pretty much every guy (and most girls, but in different ways) finds very difficult. Unfortunately the nature of Christian living is that we will always struggle with sin and will stumble. Its part of recognising our own weaknesses and turning to God for help. Ultimately He is there to help and we will never be in a situation that we cannot overcome (1 Cor 10:13). As a result, it’s actually helpful if guys talk to each other about their struggles and are accountable to each other - particularly in this area. Sin thrives in secrecy!
A verse that is helpful in terms of encouragment is Philippians 4:8 “Whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable - if anything is excellent or praiseworthy - think about such things.” This verse, while probably not stictly about lust, speaks about fixing our mind on things that are pure. So this would be a good verse to memorise and think about regularly when tempted with lust.
Another verse that is helpful to ponder and practise is 2 Peter 1:5ff “For this very reason, make every effort to add to your faith goodness; and to goodness, knowledge; and to knowledge, self-control; and to self-control, perseverance; and to perseverance, godliness; and to godliness, brotherly kindness; and to brotherly kindness, love. For if you possess these qualities in increasing measure, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ.” If you concentrate on this list of positive attributes - meditating on them and working out how to implement them - then you will be focused on godliness and not sinfulness.
Regarding what is and isn’t lust, it’s not lust to notice an attractive girl. It is lust if you dwell on that thought and take it further in your mind. This distinction is helpful, because it is impossible not to notice attractive people (unless you walk around with a bucket on your head :-). This allows you to thank God for the beautiful and amazing ways God has made people in his creation without feeling guilty about noticing attractive girls.
So I think it is wrong to imagine having sex, even with cartoon characters, and imagining people naked, as this isn’t just noticing but using people for your own satisfaction. Also, it is not filling your mind with what is true, noble, right, pure etc.
I hope this helps. Please feel free to ask more questions if you would like more information.